1. Field of the Invention
The invention relates to improved target utilization in plasma sputtering or sputter etching operations. More particularly, it relates to improved, more uniform utilization of the sputtering source material (i.e. the sputter ‘target’), so that a greater proportion of that source material can be utilized to coat substrates.
2. Description of Related Art
Sputter coating apparatus are known generally. In a typical apparatus, an energy discharge is used to excite the atoms of an inert gas, e.g. argon, to form an ionized gas or plasma. The energetic plasma is directed (accelerated) toward the surface of a sputter target by application of a magnetic field. The sputter target typically is provided in the form of a rectangular slab or sheet or plate. The plasma bombards the surface of the target, thus eroding that surface and liberating target material. The liberated target material then can be deposited onto a substrate, such as metal or plastic or glass, to provide a thin-film coating of the target material on the substrate. This process is also sometimes referred to as magnetron sputtering, due to the use of a magnetic field to support the plasma discharge.
To generate the plasma from the ground-state gas, such as argon, a cathode is applied with a negative voltage in a vacuum chamber; a separate electrode or the vacuum chamber itself may serve as anode. This way a plasma discharge is maintained by an inert gas such as argon or a mixture of inert gases with reactive gases. In one construction, e.g. illustrated in U.S. Pat. No. 5,399,253, which is assigned to the assignee of the present application, the sputter target itself is provided as the cathode. This ensures the plasma is efficiently generated immediately above the sputter target surface. Also shown in the '253 patent, the magnetic field used to direct the plasma is or can be generated by a series of magnets located behind the sputter target (cathode), facing a rear surface of the target opposite the surface where the plasma will bombard the target.
To promote more uniform erosion of the target, longitudinally-extending magnets, which extend parallel to the long axis of the target, can be moved laterally or otherwise rotated about their longitudinal axes in order to shift resulting magnetic fields laterally relative to the target; that is, in a direction that is perpendicular to the long axis of the target. The '253 patent discloses one embodiment for doing this. In that patent, a pair or pairs of oppositely-oriented (referring to their polar orientation) and longitudinally-extending permanent magnets are disposed substantially coplanar with and located within an outer permanent magnet frame that does not move. These magnets, which extend in a direction parallel to the long axis of the target, are rotatable about their respective longitudinal axes. As described in more detail in the '253 patent, the rotation of these magnets within the outer permanent magnet frame causes lateral shifting of the correspondingly induced magnetic fields (‘tunnel’ magnetic fields) at the opposite surface of the sputter target, which in turn produces more uniform plasma bombardment of that surface in the lateral direction.
But this lateral shifting of the magnetic field has virtually no effect on the portion of the plasma that bombards the longitudinal end regions of the target. This is because the longitudinally-extending magnets mentioned above do not extend the full length of the target, so their rotation has little effect on the magnetic fields in the end regions. Moreover, because the outer magnet frame does not move, the magnetic fields adjacent the longitudinal end regions of the target are relatively stagnant compared to those located in the central region of the target, which shift laterally based on rotation of the magnets as already explained. The result is that plasma bombardment in the longitudinal end regions of the target is relatively focused compared to in the central region, located between the end regions. Consequently, there tends to be relatively deep, focused erosion in the end regions, compared to the relatively more uniform erosion in the central region of the target.
The sputter target in a sputtering operation is typically replaced as soon as or before it is completely eroded (i.e. penetrated) at any location. Because relatively more focused erosion occurs in the end regions, where the magnetic fields and consequently the plasma bombardment paths are stagnant, the entire target plate usually is replaced long before erosion from the center region would warrant replacement. The result is that a substantial amount of sputter source material in the target plate is wasted, because the plate is replaced early enough so that deep erosion in the end regions does not penetrate the target. Alternatively, a step-wise target plate structure has been employed, where the target plate is relatively thicker in the end regions than in the central region. This way, the more focused erosion in the end regions is accommodated by a thicker target in those regions, and a greater proportion of the central-region material can be used because the target plate can be employed for a longer period of time. However, this step-wise structure also is undesirable. For one thing, it also results in wasted material—this time in the end regions. Another disadvantage is, that manufacturing such target is costly and that the effort for target exchange is increased.
As will be appreciated by those of ordinary skill in the art, in a conventional sputtering apparatus the strongest (deepest) point of erosion contributes relatively little of the total material sputtered from the target. Nevertheless, this point often determines the lifetime and therefore the maximum target utilization of the entire target.
Consequently, there is a need in the art to provide more uniform erosion from the end regions of a sputter target, and not just to the central region located between the end regions. Preferably, erosion (namely the depth of erosion) will be achieved both in the end regions and the central region at more nearly the same rate than using conventional equipment.